The present invention relates to a method for forming a stitch line in an upholstery member.
For example, upholstered articles, of which at least the outermost parts are formed with surface sheets, are known to be used as interior parts such as instrument panels and consoles of vehicles. One form of such upholstered articles is as follows. An upholstered article includes a base member and an upholstery member provided on the base member. Upholstery members include ones that are formed by a single layer and ones that are formed by laminating a cushion layer on the back of a surface sheet. Upholstery members typically have stitch lines, in which stitches are arranged in two or more rows on the surface of the surface sheet.
When upholstered articles are manufactured, stitch lines are formed, for example, in the upholstery member. The upholstery member with the stitch lines is then bonded to the base member.
A sewing cycle is performed to form stitch lines in an upholstery member. The sewing cycle includes feeding an upholstery member 51 in a feeding direction A, which is perpendicular to the thickness direction of the upholstery member 51 (the direction perpendicular to the sheet of FIG. 11) and reciprocating a sewing needle in the thickness direction of the upholstery member 51. The sewing cycle forms needle receiving holes 52 in the upholstery member 51.
An upper thread 53, which is fed through reciprocation of the sewing needle, is passed through the needle receiving hole 52 from the front face of the upholstery member 51 and is then caused to cross a lower thread 54, which is located on the back face of the upholstery member 51. Thereafter, the upper thread 53 is pulled out of the needle receiving hole 52 from the front face. The sewing cycle is repeatedly performed to form a stitch 55 between adjacent needle receiving holes 52, so that stitches 55 are arranged in a row to form a stitch line 56. Thread ends 55a of an adjacent pair of the stitches 55 are arranged in each needle receiving hole 52.
As the sewing needle, one with a circular cross-sectional shape at the distal end is typically used. For example, Japanese Laid-Open Patent Publication No. 2013-43571 discloses a technique for forming a stitch line by sewing an upholstery member with an upper thread and a lower thread.
Recently, to improve the appearance of upholstered articles, there has been a trend in which stitches are demanded to extend in the feeding direction A of the upholstery member 51 to make a straight stitch line 56.
However, in the above described conventional stitch line forming method, tension is applied to the lower thread 54 as indicated by arrows X in FIG. 11 to ensure the sewing quality such as the appearance. Such tension generates a force F1 that acts to rotate the thread ends 55a in the needle receiving hole 52 about a crossing part 57 of the upper thread 53 and the lower thread 54, which serves as a pivot. The force F1 moves the thread ends 55a along a curved inner circumferential surface 58 of the needle receiving hole 52, so that the thread ends 55a rotate clockwise about the crossing part 57 as viewed in FIG. 11. Such rotation occurs in each needle receiving hole 52, so that each stitch 55 is inclined relative to the feeding direction A as shown in FIGS. 10 and 12. This configuration thus fails to satisfy the above demand.